Monocrystalline diamonds, as found in nature, can be classified according to color, chemical purity and end use. The majority of monocrystalline diamonds are colored, and contain nitrogen as an impurity, and are thereby used primarily for industrial purposes; these would be classified as type Ia and Ib. The majority of gem diamonds (which are all considered “monocrystalline” diamonds) are colorless or various light colors and contain little or no nitrogen impurities; and would be classified as type IIa. Types Ia, Ib and IIa are electrical insulators. A rare form of monocrystalline diamond (classified as type IIb) contains boron as an impurity, is blue in color and is a semiconductor. In nature these characteristics are uncontrolled and therefore the color, impurity level and electrical characteristics are unpredictable and cannot be utilized to produce large volumes of specialized articles in a predictable manner.
Monocrystalline diamond provides a wide and useful range of extreme properties, including hardness, coefficient of thermal expansion, chemical inertness and wear resistance, low friction, and high thermal conductivity. Generally monocrystalline diamond is also electrically insulating and optically transparent from the ultra-violet (UV) to the far infrared (IR), with the only absorption being carbon-carbon bands from about 2.5 μm to 6 μm. Given these properties, monocrystalline diamonds find use in many diverse applications including, as heat spreaders, abrasives, cutting tools, wire dies, optical windows, and as inserts and/or wear-resistant coatings for cutting tools. The engineering and industrial uses of diamonds have been hampered only by the comparative scarcity of natural monocrystalline diamond. Hence there has been a long running quest for routes to synthesize monocrystalline diamond in the laboratory.
Synthetic monocrystalline diamonds, for industrial use, can be produced by a variety of methods, including those relying on a “high pressure method” and those involving controlled vapor deposition (CVD). Diamond produced by either the “high pressure method” or the CVD method can be produced as monocrystalline diamond or polycrystalline diamond. High pressure diamond is usually formed as micron sized crystals, which can be used as grit or loose abrasive, or set into metal or resin for cutting, grinding or other applications.
Both methods, i.e., “high pressure method” and “CVD method” make it possible to control the properties to a high degree and thereby control the properties of color, impurity level and electrical characteristics on a theoretical level. However, on a practical level, in order to manufacture useful objects by the “high pressure method”, there are limitations imposed by the presence or absence of impurities. As an example, it has been suggested that the addition of nitrogen might assist in the growth of large crystals, although the elimination of nitrogen, or the addition of boron, can make it more difficult to grow large crystals. In addition, it appears that it is not possible to make monocrystalline structures having layers of varied composition without having to remove the seed crystal from the reactor after each layer is grown, and then replacing the seed crystal in the reactor in order to grow a subsequent layer having a different composition. Moreover, large seeds cannot be accommodated in the “high pressure method”. In the CVD method, most work has been confined to production of polycrystalline diamond, as opposed to the growth and control of single crystals.
It is actually difficult and expensive to produce high quality pure monocrystalline diamond by the high pressure method. It has been shown that the addition of boron to a synthetic monocrystalline or polycrystalline diamond makes it useful for constructing a semiconductor device, a strain gauge or other electrical device although monocrystalline diamond is to be preferred. See U.S. Pat. No. 5,635,258. See also, W. Ebert, et al. “Epitaxial Diamond Schottky Barrier Diode With On/Off Current Ratios in excess of 107 at High Temperatures”, Proceedings of IEDM, pp. 419–422 (1994), Published by IEEE, and S. Sahli, et al., “Piezoelectric Gauge Factor Measured at Different Fields and Temperatures”, pp. 95–98, Applications of Diamond Films and Related Materials, A. Feldman, et al. editors, NIST Special Publications 885.
So called ‘industrial diamond’ has been synthesized commercially for over 30 years using high-pressure high-temperature (HPHT) techniques, in which monocrystalline diamond is crystallized from metal solvated carbon at pressures of about 50 to 100 kbar and temperatures of about 1800 to 2300K. In the high pressure method the crystals grow in a three dimensional manner and the crystal is all of one impurity level, except for possible discontinuities arising from fluctuations in the growth cycle. See, for example, R. C. Burns and G. Davis, “Growth of Synthetic Diamond”, pp. 396–422, The Properties of Natural and Synthetic Diamond, J. E. Field, editor, Academic Press (1992), U.S. Pat. Nos. 3,850,591 and 4,034,066.
Interest in diamond has been further increased by the much more recent discovery that it is possible to produce polycrystalline diamond films, or coatings, by a wide variety of chemical vapor deposition (CVD) techniques using, as process gases, nothing more exotic than a hydrocarbon gas (typically methane) in an excess of atomic hydrogen. CVD diamond grows two dimensionally, layer by layer and it is therefore possible to build up a bulk crystal (or plate or film) which can be of a single composition or composed of layers of many compositions (called a “structure”). CVD diamond grown in this manner can show mechanical, tribological, and even electronic properties comparable to those of natural diamond. See, for example, Y. Sato and M. Kamo, “Synthesis of Diamond From the Vapor Phase”, pp. 423–469, The Properties of Natural and Synthetic Diamond, J. E. Field, editor, Academic Press (1992). See also U.S. Patents for background; U.S Pat. Nos. 4,940,015; 5,135,730; 5,387,310; 5,314,652; 4,905,227; and 4,767,608.
There is currently much optimism that it will prove possible to scale-up CVD methods to such an extent that they will provide an economically viable alternative to the traditional high pressure methods, e.g., for producing diamond abrasives and heat spreaders. The ability to coat large surface areas with a continuous film of diamond, in turn, will open up new potential applications for the CVD-prepared materials. Today, however, the production of monocrystalline diamond by the CVD process is considerably less mature than high pressure, and the resultant materials tend to have higher defect levels and smaller sizes.
Chemical vapor deposition, as its name implies, involves a gas-phase chemical reaction occurring above a solid surface, which causes deposition onto that surface. All CVD techniques for producing diamond films require a means of activating gas-phase carbon-containing precursor molecules. This generally involves thermal (e.g., hot filament) or plasma (e.g., D.C., R.F., or microwave) activation, or the use of a combustion flame (oxyacetylene or plasma torches). Two of the more popular experimental methods include the use of a hot filament reactor, and the use of a microwave plasma enhanced reactor. While each method differs in detail, they all share features in common. For example, growth of diamond (rather than deposition of other, less well-defined, forms of carbon) normally requires that the substrate be maintained at a temperature in the range of 1000–1400 K, and that the precursor gas be diluted in an excess of hydrogen (typical CH4 mixing ratio ˜1–2 vol %).
The resulting films are usually polycrystalline (unless a monocrystalline diamond seed is provided) with a morphology that is sensitive to the precise growth conditions. Growth rates for the various deposition processes vary considerably, and it is usually found that higher growth rates can be achieved only at the expense of a corresponding loss of film quality. Quality is generally taken to imply some measure of factors such as the ratio of sp3 (diamond) to sp2-bonded (graphite) carbon in the sample, the composition (e.g. C—C versus C—H bond content) and the crystallinity. In general, combustion methods deposit diamond at high rates (typically 100 μm/hr to 250 μm/hr), but often only over very small, localized areas and with poor process control, thereby leading to poor quality films. In contrast, the hot filament and plasma methods tend to provide have much slower growth rates (0.1–10 μm/hr), but produce high quality films.
One of the great challenges facing researchers in CVD diamond technology is to increase the growth rates to economically viable rates, (to the level of 100+ μm/h, or even one or more mm/hr) without compromising film quality. Progress continues to be made in the use of microwave deposition reactors, since the deposition rate has been found to scale approximately linearly with applied microwave power. Currently, the typical power rating for a microwave reactor is ˜5 kW, but the next generation of such reactors have power ratings up to 50–80 kW. This gives a much more realistic deposition rate for the diamond, but for a much greater cost, of course.
Thermodynamically, graphite, not diamond, is the stable form of solid carbon at ambient pressures and temperatures. The fact that diamond films can be formed by CVD techniques is inextricably linked to the presence of hydrogen atoms, which are generated as a result of the gas being ‘activated’, either thermally or via electron bombardment. These H atoms are believed to play a number of crucial roles in the CVD process:                They undergo H abstraction reactions with stable gas-phase hydrocarbon molecules, producing highly reactive carbon-containing radical species. This is important, since stable hydrocarbon molecules do not react to cause diamond growth. The reactive radicals, especially methyl, CH3, can diffuse to the substrate surface and react, forming the C—C bond necessary to propagate the diamond lattice.        H-atoms terminate the ‘dangling’ carbon bonds on the growing diamond surface and prevent them from cross-linking, thereby reconstructing to a graphite-like surface.        Atomic hydrogen etches both diamond and graphite but, under typical CVD conditions, the rate of diamond growth exceeds its etch rate, while for other forms of carbon (graphite, for example) the converse is true. This is believed to be the basis for the preferential deposition of diamond rather than graphite.        
One major problem receiving a lot of attention is the mechanism of heteroepitaxial growth, that is, the initial stages by which diamond nucleates upon a non-diamond substrate. Several studies have shown that pre-abrasion of non-diamond substrates reduces the induction time for nucleation and increases the density of nucleation sites. Enhanced growth rates inevitably follow since formation of a continuous diamond film is essentially a process of crystallization, proceeding via nucleation, followed by three-dimensional growth of the various microcrystallites to the point where they eventually coalesce.
The abrasion process is usually carried out by polishing the substrate with an abrasive grit, usually diamond powder of 0.1 μm to 10 μm particle size, either mechanically or by ultrasonic agitation. Regardless of the abrasion method, however, the need to damage the surface in such a poorly defined manner, prior to deposition, may severely inhibit the use of CVD diamond for applications in areas such as the electronics industry, where circuit geometries are frequently on a submicron scale. This concern has led to a search for more controllable methods of enhancing nucleation, such as ion bombardment. Ion bombardment can be performed in a microwave deposition reactor, by simply adding a negative bias of a few hundred volts to the substrate and allowing the ions to (i) damage the surface, (ii) implant into the lattice, and (iii) form a carbide interlayer.
These methods are in direct contrast to the production of monocrystalline diamond by CVD, e.g., where a polished monocrystalline diamond is used as a seed crystal and the structure of that seed crystal is reproduced in the new monocrystalline diamond grown on the seed. The resulting monocrystalline diamond has superior properties to polycrystalline diamond for most industrial, optical, electronic and consumer applications.
A variety of methods have been described for use in preparing synthetic diamonds. See, for example, U.S. Pat. Nos. 5,587,210, 5,273,731 and 5,110,579. Most of the scientific research effort into CVD diamond technology has been concentrated within the past five years yet, already, some of the more immediate applications, such as cutting tools and heat spreaders, have reached the market-place. Several problems need to be addressed and over come before this technology begins to make a significant impact however. Growth rates need to be increased (by one or more orders of magnitude) without loss of film quality. Deposition temperatures need to be reduced by several hundred degrees, allowing low melting point materials to be coated and to increase the number of substrates onto which adherent diamond films can be deposited. A better understanding of the nucleation process is required, hopefully leading to an elimination of the poorly controlled pre-abrasion step. Substrate areas need to be scaled up, again without loss of uniformity or film quality. For electronic applications, single crystal diamond films are desperately needed, along with reliable techniques for patterning and controlled n- and p-type doping.
On a related subject, diamond has the highest thermal conductivity of any known material, with a value on the order of five times that of copper metal. High thermal conductivity is a very important property for many applications because it permits heat to be removed rapidly from a narrow source and spread to a larger area where it can be completely removed from an operating system.
In the area of materials fabrication (cutting), heat is generated at the cutting tip or edge of a tool as a result of the cutting process. If that heat is not removed the temperature of the cutting tool increases to the point that it degrades by oxidation, corrosion or fracturing and the tool becomes unusable. Furthermore, as the tool is degrading, the quality and precision of the part being fabricated degrades significantly. When a cutting tool is made of diamond, the high thermal conductivity of diamond, heat from the cutting tip or edge is rapidly removed from that tip or edge to the tool holder and the temperature of the cutting tip or edge runs significantly cooler than comparable tools made from other materials such as carbides, oxides, nitrides or borides. Therefore, diamond tools can generally run longer and provide higher quality manufactured parts over a longer period of time than alternative cutting materials (cutting tool patents). In a similar manner, wire dies are made of diamond because they have great resistance to wear and because the heat of drawing the wire can rapidly be dissipated from the wire. This results in a longer life for the wire die and higher quality of wire for a longer length of wire than can be obtained with alternative wire die materials. (wire die patents) point or surface which is generating heat, thereby diamond cutting tools and wire dies to conduct heat away from the cutting surface in tools and the wear surface in wire dies and promotes longer life by reduced wear and enables a higher quality part or wire to be fabricated throughout the life of the tool or wire die.
In windows for high power lasers, thermal lensing occurs when light is partially absorbed by the lens material and the lens material heats causing a change in index of refraction of the lens material. Since the heat generated by the laser beam must be dissipated to the outer surfaces of the sense there will be a gradient in the index of refraction of the material and that will cause the laser beam to be distorted or to focus or defocus in an uncontrolled manner. Such uncontrolled distortion will result in uncontrolled cutting or welding in high power laser fabrication equipment and limit the useful power and thereby the number of applications to which such lasers can be used. The same problems arise in the use of high power lasers for communications, fusion power or other applications well known to those who are engaged in the art.
It is apparent that the use of diamond windows in high power laser systems is highly desirable and would lead to higher power laser cutters and welders and other applications such as communications. It is also apparent that even higher thermal conductivity diamond would result in higher power lasers becoming feasible. It is further apparent that breakdown and damage of the diamond window will be governed by how rapidly heat can be removed from the window, thereby higher thermal conductivity diamond windows would be expected to experience a reduced failure rate from breakdown and damage.
In semiconductor devices such as solid state laser and high power microwave devices a high level of heat is generated in a very small area. This heat must be removed or the device temperature will rapidly rise to the level that the device will cease to operate properly or fail catastrophically. This problem can be alleviated by attaching the semiconductor device a diamond plate which rapidly removes heat form the small area of the device and spreads it to a larger area of a cooling fin or cooling device (P. Hui, et al, Temperature Distribution in a Heat Dissipation System Using a Cylindrical Diamond Heat Spreader on a Copper Block, J. Appl. Phys. 75 (2), 15 Jan. 1994). Diamond has also been suggested for use to cool three dimensional arrays of semiconductor devices or IC's to produce very high speed three dimensional computers where the stacks of chips are to be cooled by contact with diamond plates. (R. Eden, Applications in Computers, Handbook of Industrial Diamonds and Diamond Films, pp 1073–1102, Editors, Mark Prelas, Galina Popovici and Louis Bigelow, Marcel Decker, NY, 1998).
In all of these devices and cutting tools, the performance and lifetime is directly related to the temperature of the active part of the device and tool. The operating temperature of the active part of the device/tool is directly related to the thermal conductivity of the diamond heat being used. However, the thermal conductivity (TC) of diamond is dramatically effected by impurities, crystal defects and by polycrystallinity. Therefore the performance of a diamond tool or a diamond cooled semiconductor will be directly related to the thermal conductivity of the diamond used. (see M. Seal, “High Technology Applications of Diamond”, pp. 608–616, The Properties of Natural and Synthetic Diamond Edited by J. E. Field, Academic Press (1992)).
Polycrystalline diamond typically has the lowest thermal conductivity, nitrogen doped single crystal is higher, followed by pure diamond which has the highest. The highest thermal conductivity natural diamond is type IIa which contains little to no nitrogen and has values of thermal conductivity of 2000 to 2500 watt/meter degree Kelvin (W/mK). (see V. I Nepsha, “Heat Capacity, Conductivity, and Thermal Coefficient of Expansion”, pgs. 147–192, Handbook of Industrial Diamond and Diamond Films, M. A. Prelas, G. Popovici, and L. K. Bigelow, Editors, Marcel Dekker, Inc. (1998)). Numerous measurements of natural diamond and synthetic diamond produced by the high temperature high pressure method showed that this value of 2000 to 2500 was usually the highest attainable thermal conductivity, and is the accepted value to this day. A TC value of about 2200 was also attained in high quality polycrystalline diamond. See, e.g., CVD Diamond: a New Engineering Material for Thermal, Dielectric and Optical Applications, R. S. Sussman, et al., Industrial Diamond Review, 58(578):69–77 (1998).
Thermal conductivity in diamond occurs by phonon-phonon transfer and thermal conductivity is controlled by the mean free path (l) those phonons in the diamond crystal. Therefore any property of diamond which causes a variation in the mean free path of a phonon will cause a variation in thermal conductivity of the diamond. Scattering of phonons reduces the mean free path and phonon scattering can be caused phonon-phonon interactions (ppi), grain boundaries (gb), dislocations (dis), vacancies (vac), impurities (imp), isotopes (iso) and other mechanisms including voids (othr). The mean free path of a phonon is given by the equation:1/l=1/l(ppi)+1/l(gb)+1/l(dis)+1/l(vac)+1/l(imp)+1/l(iso)+1/l(othr)
For an in-depth summary of the theory see U.S. Pat. No. 5,540,904. Carbon exists in three isotopes 12C, 13C and 14C. 12C is present at levels of 99 percent in natural diamond, 13C and 14C is so low and radioactive so that it is used for dating in geological or archeological sites. Application of the aforementioned theory has led to significant improvements of the thermal conductivity of diamond crystals and polycrystalline diamond by reducing the amount of carbon 13 (13C) in these materials and by increasing the grain size in polycrystalline diamond thereby reducing the volume of grain boundaries. By reduction of the 13C content of single crystal and polycrystalline diamond from 1.1% (as found in natural diamond and naturally occurring diamond precursors) to 0.001% 13C the thermal conductivity at room temperature was found to increase from 2000 W/mK (in natural isotope diamond) to 3300 W/mK in isotopically enriched diamond (U.S. Pat. Nos. 5,540,904, 5,360,479 and 5,419,276).
It was also found that the thermal conductivity of diamond could be altered by changing the purity the diamond starting materials with respect to the carbon isotopes. For instance, when diamond crystals or polycrystalline diamond was produced which was 99.999% pure with respect to the 12C isotope, the thermal conductivity at room temperature increased to 3300 W/mK. It was also concluded that this was indeed the highest thermal conductivity possible and that since this high thermal conductivity was also observed in polycrystalline diamond, crystalline properties such as grain boundaries were not a major loss of thermal conductivity. Theoretical analysis of the thermal conductivity of diamond was conducted by (P. G. Klemens, Solid State Physics: Advances in Research and Applications, edited by R. Seitz and D. Tumbill (Academic, New York, 1958) Vol. 7) which showed agreement with the above cited work in that it predicted that high thermal conductivity would occur in isotopically enriched diamonds. However, this work also predicted that the thermal conductivity of pure natural isotope concentration diamond should also be 3300 W/mK at room temperature. Since no natural diamond or synthetic diamond has been found which has a thermal conductivity higher than 2200 W/mK it was concluded by those skilled in the art that either, the theory was wrong or there is some yet unaccounted factor which degrades the thermal conductivity of natural isotope concentration diamond.
The ability to increase the thermal conductivity of single crystal diamond by over 50% from 2300 W/mK to 3300 W/mK or more would offer significant performance enhancements for diamond cutting tools, diamond wire dies, high power laser windows, high power semiconductor devices such as lasers, microwave devices and three dimensional computers or circuits. The advantages of enhanced thermal conductivity diamond have, to date however, not been applicable to any commercial application because the cost of producing 12C enhanced precursor gasses (typically methane gas) is prohibitively high compared to the cost of natural isotope gasses. Typically, the cost of 12C enriched precursors, such as methane gas, is $75 to $200 per gram, compared with the cost of unenriched methane at less than $0.01 per gram. Since only 1 to 2% of the methane is converted to diamond this would result in a cost of materials for enhance thermal conductivity of $7,500 to $20,000 per gram of diamond crystal produced and the cost for natural isotope raw materials at less than $1 per gram of diamond produced. This high cost far overshadows the advantages in tool, wire die, window or device performance obtained and would permit such enhanced thermal conductivity diamond to be used in only the most demanding and cost tolerant and low volume specialty applications.